Bio Practice Questions and Answers 2

Describe how lysosomal enzymes are targeted to the lysosome, starting with the synthesis of the proteins on the ER membrane. Be sure to explain the role of mannose-6-phosphate.

1. As lysosomal enzymes are translated on the ER membrane, N-linked glycosylation adds an oligosaccharide chain to the proteins. 2. The lysosomal enzymes are transported in vesicles to the cis-Golgi network. 3. While progressing through the Golgi (cis -> medial -> trans), the oligosaccharide chains on the lysosomal enzymes are modified with mannose-6-phosphate (M-6-P) to identify them as enzymes destined for the lysosome. 4. In the trans-Golgi network (TGN), mannose-6-phosphate receptors collect the lysosomal enzymes using adaptin-1 and a clathrin coat (clathrin is a type of coat protein), forming a transport vesicle. 5. The transport vesicle travels to the early endosome, where the slightly acidic pH separates the lysosomal enzymes from the mannose-6-phosphate receptors. 6. The mannose-6-phosphate receptors are packaged into vesicles and returned to the trans-Golgi network (recycled). 7. The lysosomal enzymes travel to the lysosome, where the acidic pH enables the enzymes to hydrolyze biological molecules. Remember: the Golgi modifies and sorts proteins.

Place the following events in order and name the stage of mitosis in which they occur. A. alignment of the chromosomes at the spindle equator B. attachment of spindle microtubules to chromosomes C. breakdown of nuclear envelope D. pinching of cell in two E. separation of two centrosomes and initiation of mitotic spindle assembly F. re-formation of the nuclear envelope G. condensation of the chromosomesH. separation of sister chromatids

1. G (prophase) 2. E (prophase) 3. C (prometaphase) 4. B (prometaphase) 5. A (metaphase) 6. H (anaphase) 7. F (telophase) 8. D (cytokinesis)

Which of the following statements is FALSE? A. Cdc25 dephosphorylation of Wee1 activates the kinase, promoting the G2/M transition. B. Phosphorylation of mitotic Cdk by the inhibitory kinase (Wee1) makes the Cdk inactive. C. Inhibiting the Cdc25 phosphatase will delay the G2/M transition. D. The activating phosphatase (Cdc25) removes the phosphates from mitotic Cdk that were added by Wee1, so that M-Cdk will be active. E. More than one of the above.

A is correct. The Cdc25 phosphatase acts to dephosphorylate M-Cdk and does not dephosphorylate Wee1. Furthermore, Wee1 activation would inhibit M-Cdk activity, which would inhibit (and not promote) the G2/M transition.

Indicate whether the following statements about preparations for chromosome segregation are TRUE or FALSE. If false, change a single noun to make the statement true. A. Sister chromatids are held together by condensins from the time they arise by DNA replication until the time they separate at anaphase. B. Cohesins are required to make the chromosomes more compact and thus to prevent tangling between different chromosomes. C. Microtubule-dependent motor proteins and microtubule depolymerization are mainly responsible for the organized movements of chromosomes during mitosis. D. Centrosome duplication during M phase is triggered by S-Cdk. E. Centrosomes form a unipolar array during mitosis.

A. FALSE. Sister chromatids are held together by cohesins from the time they arise by DNA replication until the time they separate at anaphase. B. FALSE. Condensins are required to make the chromosomes more compact and thus to prevent tangling between different chromosomes. C. TRUE. D. FALSE. Centrosome duplication during interphase is triggered by S-Cdk. E. FALSE. Centrosomes form a bipolar array during mitosis. Centrosomes form a unipolar array during interphase.

Following S phase, an animal cell will enter _______.

A. M phase B. G1 C. G2 D. GO

A. What causes the concentration of M-cyclin to rise gradually and fall rapidly? B. Why does M-Cdk remain inactive as the concentration of M-cyclin increases initially? C. Why does the activity of M-Cdk increase so rapidly?

A. M-cyclin concentrations rise gradually because the gene must be transcribed and the protein synthesized by translation. M-cyclin concentrations fall when the Anaphase Promoting Complex (APC/C) polyubiquitylates the cyclin, marking it for destruction by the proteasome. B. M-Cdk remains inactive when M-cyclin concentrations initially rise because M-cyclin concentrations are insufficient to activate M-Cdk (M-cyclin must reach a threshold concentration before M-Cdk activity increases). Inhibitory kinase Wee1: M-Cdk is inactivated by an inhibitory phosphorylation from the Wee1 inhibitory kinase. Activating phosphatase Cdc25: M-Cdk is activated by the removal of the inhibitory phosphate by the Cdc25 activating phosphatase. C. Positive feedback loops result in the steep rise in M-Cdk activity. Active M-Cdk complexes phosphorylates and activates Cdc25. Active M-Cdk complexes phosphorylates and inactivates the inhibitory kinase Wee1. Both positive feedback loops lead to increased M-Cdk activity.

The G1/S checkpoint __________ . A. causes cells to proceed through S phase more quickly. B. involves the degradation of p53. C. is activated by errors caused during DNA replication. D. involves the inhibition of cyclin-Cdk complexes by p21

Correct answer is D

A. Describe the structure of the bipolar attachment that forms between microtubules and chromosomes during prometaphase. Use the top figure below to guide your description. Hint: a wordlist is included below the figure. B. During prometaphase, microtubules bind randomly with the kinetochore on the centromere region of chromosomes. Why is bipolar ("bi-oriented") attachment favored?

A. See image B. Bipolar attachment is favored over other attachments because only bipolar attachment results in tension on the kinetochore, which stabilizes the interaction between the kinetochore and microtubules. Microtubule attachments to just one kinetochore or to inappropriate kinetochores do not create tension and are therefore unstable.

Which of the following statements about the cell cycle is FALSE? A. Once a cell decides to enter the cell cycle, the time from start to finish is the same in all eukaryotic cells. B. An unfavorable environment can cause cells to arrest in G1. C. A cell has more DNA during G2 than it did in G1. D. None of the above.

A. is FALSE. Also important: differences in cell cycle length are due to differences in the time spent in G1 phase.

Which of the following precede the re-formation of the nuclear envelope during M phase in animal cells?A. assembly of the contractile ring B. decondensation of chromosomes C. reassembly of the nuclear lamina D. transcription of nuclear genes

A. is correct. The contractile ring in an animal cell begins to assemble in anaphase. The chromosomes do not decondense, the lamina does not re-form, and transcription does not begin until the formation of the nuclear envelope is complete and nuclear proteins have been imported through the nuclear pores.

At the end of DNA replication, the sister chromatids are held together by the _______ .

A. kinetochores B. securins C. cohesins D. histones

Sister chromatid separation occurs because __________ are ubiquitylated by the anaphase promoting complex (APC/C). A. securins B. cohesins C. kinetochores D. condensins

A. securins The anaphase promoting complex (APC/C) initiates sister chromatid separation by polyubiquitylating securin, which leads to the degradation of securin by the proteasome. The degradation of securin results in activation of separase. Separase then cleaves the cohesins that hold the sister chromatids together.

Identify A-E in the figure below. Optional follow-up questions: Which phase of mitosis is depicted in this figure? Which component(s) in the figure are duplicated during interphase?

A. spindle pole B. duplicated chromosome (sister chromatids) C astral microtubules D kinetochore microtubules E interpolar microtubules Optional follow-up answers: The figure shows a cell during metaphase. Chromosomes (B) and the centrosome (A) are duplicated during interphase.

The activity of a protein can be regulated by the addition and removal of phosphate groups. _____________ are enzymes that phosphorylate (add phosphate groups) to a target protein. _______________ are enzymes that dephosphorylate (remove phosphate groups) from a target protein. Phosphorylation _______________________ depending on the individual protein. Dephosphorylation ____________________ .

Answer 1: Kinases Answer 2:Phosphatases Answer 3: may increase or decrease activity depending on the individual protein. Answer 4: may increase or decrease activity depending on the individual protein.

Cdk activity changes during the cell cycle, in part because ________ . A. the Cdks phosphorylate each other. B. the Cdks activate the cyclins. C. Cdk degradation precedes entry into the next phase of the cell cycle. D. cyclin concentrations change during the cycle.

Answer is D. Cyclin concentrations fluctuate throughout the cell cycle and are required for Cdk activity. M-cyclins are degraded in a cell-cycle-dependent fashion (not Cdks), and they are required for Cdk activity. Cdks do not phosphorylate each other. The Cdks do not activate the cyclins.

Irradiated mammalian cells usually stop dividing and arrest at a G1/S checkpoint. Place the following events in the order in which they occur after DNA is damaged. X. production of p21 Y. inhibition of cyclin-Cdk complexes Z. accumulation and activation of p53 A. X, Z, Y B. Z, X, Y C. Y, X, Z D. Y, Z, X E. none of the above

B is correct

What causes the concentrations of S-cyclin and M-cyclin to change throughout the cell cycle? A. phosphorylation and dephosphorylation B. protein synthesis and proteolysis C. S-Cdk and M-Cdk activity D. More than one of the above.

B is correct. Protein synthesis and proteolysis cause the concentration of cyclins to change through the cell cycle. Cyclin concentrations increase due to the transcription of cyclin genes and the synthesis of cyclin proteins through translation. Cyclin concentrations decrease due to degradation of the cyclin by proteolysis. A is incorrect. Phosphorylation and dephosphorylation is incorrect. Cyclin concentrations change throughout the cell cycle. Phosphorylation and dephosphorylation are mechanisms that influence protein activity, not concentration. C is incorrect. Cdk-cyclin complexes do not influence the concentration of cyclins.

Which of the following statements is FALSE? A. DNA synthesis begins at origins of replication. B. The loading of the origin recognition complexes (ORCs) is triggered by S-Cdk. C. The phosphorylation and degradation of Cdc6 help to ensure that DNA is replicated only once in each cell cycle. D. DNA synthesis can only begin after prereplicative complexes assemble on the ORCs.

B. is FALSE. S-Cdk helps guarantee that DNA replication occurs only once during each cell cycle. Cdc6 guides the assembly of the prereplicative complex at an origin. By phosphorylating Cdc6, S-Cdk inactivates it, so that once an origin fires and replicates the DNA, Cdc6 cannot reinitiate DNA replication. S-Cdk phosphorylates and activates DNA helicases.

A mutant yeast strain stops proliferating when shifted from 25°C to 37°C. When these cells are analyzed at the two different temperatures, using a machine that sorts cells according to the amount of DNA they contain, the graphs in the figure below are obtained. Which of the following would NOT explain the results with the mutant? A. inability to initiate DNA replication B. inability to begin M phase C. inability to activate proteins needed to enter S phase D. inappropriate production of a signal that causes the cells to remain in G1

B. is correct. At 37°C, the cells all have one genome-worth of DNA, meaning that they have not replicated their DNA and therefore have not entered S phase. Cells that are unable to begin M phase should have two genomes-worth of DNA, as they would have completed DNA replication and arrested in G2.

You create cells with a version of the origin recognition complex (ORC) that cannot be phosphorylated by S-Cdk and thus cannot be inactivated. Which of the following statements describes the likely consequence of this change in ORC? A. Cells will enter S phase prematurely. B. Cells will replicate some regions of the genome more than once in a cell cycle. C. ORC will be unable to bind to DNA. D. DNA helicases will not be able to open up the double helix at the replication origin.

B. is correct. If ORC is not properly inactivated (and thus remains active), ORC could bind to DNA and reassemble a prereplicative complex on a DNA molecule that has already undergone replication. The assembly of an inappropriate prereplicative complex would lead to an inappropriate reinitiation of replication.

Chromosomes vs. Centromere

Be careful: it is easy to get the words centrosome and centromere confused! Centrosomes are the principal microtubule-organizing center in animal cells. Centromeres are the region on each chromosome where there are no genes and where microtubules of the spindle attach, via kinetochore proteins, during mitosis. Each sister chromatid has its own centromere!

The cell-cycle control system relies on an increase in the activity of______________["APC/C", "microtubules", "M-Cdk", "S-Cdk"] to trigger DNA replication. Inactivation of ________________ ["APC", "microtubules", "M-Cdk", "S-Cdk"] is required to transition from metaphase to anaphase.

Blank 1: S-Cdk Bank 2: M-Cdk

Motor protein A is ___________. The movement of the motor on a/an ________ microtubule which _______ the centrosome toward the cell surface. Motor protein B is a___________ . The movement of the motor on a/an __________microtubule __________the centrosomes apart.

Blank 1: dynein Blank 2: astral Blank 3: pulls Blank 4: kinesin-5 Blank 5: interpolar Blank 6: pushes

Which of the following statements about cytokinesis is FALSE? A. In plant cells small membrane vesicles derived from the Golgi apparatus deliver new cell-wall material for the new wall of the dividing cell. B. In animal cells contraction of an actin-based contractile ring generates two daughter cells. C. In plant and animal cells secretory trafficking to the phragmoplast contributes to cytokinesis. D. None of the above.

C. is FALSE. Secretory trafficking to the phragmoplast only occurs during cytokinesis in plant cells.

Which of the following statements about the anaphase-promoting complex (APC/C) is FALSE? Follow-up: describe the role of the APC/C in the spindle-assembly checkpoint. A. It promotes the degradation of proteins that regulate M phase. B. It inhibits M-Cdk activity. C. It is activated by polyubiquitylation. D. More than one of the above.

C. is FALSE. The APC/C is a small protein complex that attaches ubiquitin molecules to M-cyclin. The resulting polyubuitylated M-cyclin is then targeted for destruction by the proteasome. Note: Polyubiquitylation is a common signal that marks a protein for destruction by the proteasome. A. is true. The APC/C promotes the degradation of M-cyclin. B. is true. M-Cdk is inactive when it is not bound to M-cyclin. Thus, by promoting the degradation of M-cyclin, the APC/C inhibits M-Cdk activity. Follow-up answer: The APC/C acts at the spindle-assembly checkpoint through ubuquitylation of (1) M-cyclin and (2) securin, leading the degradation of M-cyclin and securin by the proteasome. The degradation of M-cyclin inactivates M-Cdk. The degradation of securin allows separate to degrade the cohesins holding the chromosomes together, initiating the start of Anaphase.

Actively recall what you learned about cell cycle checkpoints during lecture and complete the following questions without using your notes. A. What are cell cycle checkpoints and why are they important? B. List the three major cell cycle checkpoints and briefly describe their function (what does each checkpoint "check for"?).

Checkpoints regulate a cell's progression through the cell cycle that ensures one process is complete before the next process can begin. In the racecar analogy, they are the brakes that can temporarily halt progression through the cell cycle. G1/S checkpoint Is DNA damaged? (sends a signal to halt progression through the cell cycle until DNA can be repaired) Is the environment favorable? (cells do not divide without receiving the proper extracellular signals) DNA damage leads to inhibition of S-Cdk, blocking entry to S phase and providing time to repair DNA damage DNA damage --> activates p53 --> p53 is a transcription factor leading to expression p21 gene and synthesis of p21 protein --> p21 binds and inhibits S-Cdk. G2/M phase checkpoint Is DNA replication complete? Is DNA damaged? Entry to mitosis is blocked by inhibiting M-Cdk. M-Cdk is inactive when phosphorylated by the Wee1 kinase (Wee1 = inhibitory kinase). M-Cdk is active when Cdc25 phosphatase removes the inhibitory kinase (Cdc25 = activating phosphatase). Spindle assembly checkpoint Are chromosomes properly attached to mitotic spindle? Prevents exit from metaphase until chromosomes properly attached to spindle (improper attachment would lead to unequal distribution of duplicated DNA between daughter cells). Unattached kinetochores send chemical signals inhibiting the anaphase-promoting complex (APC/C). When chromosomes are properly attached to the spindle, inhibitory signals are no longer sent and the APC/C is active. Active APC/C ubiquinates securin, allowing separase to cleave cohesin holding chromosomes together, initiating anaphase. The APC/C also ubiquinates M-cyclin, which allows exit from mitosis.

Condensins ___________ . A. are degraded when cells enter M phase. B. assemble into complexes on the DNA when phosphorylated by M-Cdk. C. are involved in holding sister chromatids together. D. bind to DNA before DNA replication begins.

Correct Answer is B

What would be the most likely outcome of repeated cell cycles consisting of S phase and M phase only? A. The cells would not be able to replicate their DNA. B. The mitotic spindle could not assemble. C. The cells would get larger and larger. D. The cells produced would get smaller and smaller.

Correct answer is D. The cells produced would get smaller and smaller, as they would not have sufficient time to double their mass before dividing.

A. What are cyclin-dependent kinases (Cdks) and cyclins? How do cyclin-Cdk complexes drive the orderly progression of events through the cell cycle? B. How does the activity of cyclin-Cdk complexes change throughout the cell cycle. What causes this change?

Cyclin-dependent kinases (Cdks) are protein kinases that phosphorylate target proteins involved in different parts of the cell cycle. Cdks must be bound to a cyclin to be active. Cyclins are proteins that have no enzymatic activity themselves, but Cdks must be bound to a cyclin to be active. Cyclins (1) activate Cdks and (2) activate cell cycle substrate that Cdks phosphorylate. Cdk-cyclin complexes (Cdks bound to cyclin) trigger different events of the cell cycle by phosphorylating target proteins involved in different parts of the cell cycle. If you thought of the cell cycle as a race car, Cdk-cyclin complexes are the accelerator pedal that move the cell forward through the cell cycle. B. The activity of cyclin-Cdk complexes rises and falls during the cell cycle due to changes in cyclin concentrations, which fluctuate in a cyclical fashion during the cell cycle (hence the name cyclin). Cdk concentrations do not change during the cell cycle.

Which of the following statements about Anaphase A and B is TRUE? A. Anaphase A must be completed before anaphase B can take place. B. In cells in which anaphase B predominates, the spindle will elongate much less than in cells in which anaphase A dominates. C. In anaphase A, both kinetochore and interpolar microtubules shorten. D. In anaphase B, microtubules associated with the cell cortex shorten.

D. is correct. Anaphase A and anaphase B generally occur at the same time. In cells in which anaphase B predominates, the spindle will elongate more than in cells in which anaphase A predominates. In anaphase A, only the kinetochore microtubules shorten. Anaphase A: chromosomes are pulled poleward by the shortening of the kinetochore microtubules. Microtubule depolymerization at the kinetochore pulls the chromosome to the pole. Microtubule depolymerization is like unpeeling a banana and the depolymerization provides the force to move the kinetochore and chromosome toward the pole. Anaphase B: poles are pushed and pulled apart. Two-headed kinesin-5 (plus-end directed motor) slides interpolar microtubules from opposite poles, pushing poles apart. This requires interpolar microtubules to grow at the plus ends to maintain overlap as they slide apart. Dynein (minus-end directed motor) is located at the cell periphery and pulls on the astral microtubules, pulling the poles apart.

Consider what you learned about the cell cycle. What main molecular difference would you expect to see between cells in a G0 state and cells that have simply paused in G1?

In G0, the cell-cycle control system is partly dismantled, so that some of the Cdks and cyclins are not present. Cells paused in G1, by contrast, still contain all the components of the cell-cycle control system. Whereas the latter cells can rapidly progress through the cycle when conditions are right, G0 cells need to synthesize the missing cell-cycle control proteins so as to reenter the cycle, which usually takes 8 hours or more. Cells in the G0 state do not divide but can reenter the cell cycle. A terminally differentiated cell cannot reenter the cell cycle and dismantle the cell-cycle control system. Ex: many specialized cells like neurons.

Why should it be that drugs such as colchicine, which inhibit microtubule polymerization, and drugs such as Taxol®, which stabilize microtubules, both inhibit mitosis?

Mitosis requires that the spindle microtubules behave dynamically—continuously polymerizing and depolymerizing—to probe the cell cortex, to seek attachments to kinetochores, and to segregate the chromosomes. Static microtubules are unable to do any of these things.

Actively recall the key points about the cell division cycle from lecture. Without looking at your notes, briefly describe the major events during the four phases of the cell cycle (aka, the cell division cycle). What is the outcome of one round of the cell cycle?

The cell cycle describes the orderly progression of events in which a cell grows, replicates its DNA, and equally divides its DNA and organelles to produce two daughter cells. (One round of the cell cycle produces two daughter cells.) Four phases of the cell cycle: G1 phase ("gap 1"): growth, preparing for S phase. *Important: growth means the cell is synthesizing proteins, lipids, etc., - essentially doubling its mass and organelles. S phase ("synthesis"): DNA replication G2 phase ("gap 2"): growth, preparing for M phase M phase: mitosis (nuclear division) and cytokinesis (cytoplasmic division) G1, S, and G2 are all part of interphase.

The figure below shows a living cell from the lung epithelium of a newt at different stages in M phase. Order these light micrographs into the correct sequence and identify the stage in M phase that each represents.

The correct order is listed as follows, with stages in parentheses: E (prophase), D (prometaphase), C (metaphase), A (anaphase), F (telophase), and B (cytokinesis).